1. Field of the Invention
The present invention relates to a product structure of a display device that includes one or both of a transparent protective plate and a transparent substrate having a touch panel function which are disposed on a viewing side of a display panel.
2. Description of the Related Art
Many of display devices for home-use televisions and personal computers and information display devices installed in public facilities are liquid crystal display devices or organic electroluminescence display devices. Liquid crystal display devices are employed in a particularly wide range of products from as small ones as 2-inch monitors for cellular phones and the like to 60-inch or larger television monitors.
Most liquid crystal display devices include a thin film transistor (TFT) substrate and a color filter (CF) substrate which are 0.2 to 0.7 mm-thick, between which liquid crystal is held. Some cellular phones, digital cameras, small-sized information terminals, and other devices where a liquid crystal panel surface is frequently touched during use therefore have a transparent protective plate in front of a liquid crystal panel to prevent external mechanical forces from disturbing displayed data or breaking the liquid crystal panel.
In personal computer monitors, home-use televisions, and other similar appliances, too, where the liquid crystal panel surface is touched less frequently, the liquid crystal panel itself could be shattered if something like tableware or a toy hits the liquid crystal panel surface with a large enough force. Therefore, similarly to cellular phones, digital cameras, small-sized information terminals, and the like, some of such products as personal computer monitors and home-use televisions have a transparent protective plate 2 in front of a liquid crystal panel 1 as illustrated in FIG. 3 in order to prevent damage to the liquid crystal panel.
However, disposing the transparent protective plate 2 in front of the liquid crystal display panel 1 causes reflection of light at an interface between materials having different optical characteristics, specifically, refractive indices. In the case of a structure illustrated in FIG. 3, reflection of light is observed at an interface between the protective plate 2 and air existing in front of the protective plate 2, an interface between the protective plate 2 and an air layer 8 located behind the protective plate 2, and an interface between a first polarization plate 4 and the air layer 8. The reflection significantly impairs visibility of a displayed image especially in a bright environment.
In recent years, devices that have a touch panel functioning as an input device on the front side (viewing side) of the display panel are rapidly gaining popularity, because those devices may be operated easily and intuitively by touching a switch or other icons displayed on the display panel to be manipulated.
Touch panels are classified by operating principle into the following types.
[Resistive Type]
As illustrated in FIG. 4, a resistive touch panel has a structure in which transparent conductive films 21 (indium tin oxide: ITO) are formed on opposing surfaces of an upper polyethylene terephthalate (PET) film 23 and an underlying glass substrate 20, and the PET film 23 and the glass substrate 20 are then bonded together with spacers 24 interposed therebetween. As illustrated in FIG. 5, touch panel input is registered by depressing the PET film 23 with a finger 14, a pen, or the like and thus bringing the transparent conductive films 21 which are formed on the PET film 23 and the glass substrate 20, respectively, into contact with each other. The spacers 24, which are insulators disposed between the transparent conductive films 21 formed on the PET film 23 and the glass substrate 20, respectively, prevent a short circuit between the upper and lower transparent conductive films 21 when no input is made.
Resistive touch panels are generally classified into “matrix resistive type” and “analog resistive type.” In a matrix resistive touch panel, the transparent conductive film 21 on the PET film 23 and the transparent conductive film 21 on the glass substrate 20 are shaped like slips and arranged to be orthogonal to each other, and a touched point is detected from an X coordinate and a Y coordinate where the slip-shaped transparent conductive films 21 are brought into contact with each other. In an analog resistive touch panel, the transparent conductive films 21 are formed all over the opposing surfaces of the PET film 23 and the glass substrate 20, one of the transparent conductive films 21 constitutes an X coordinate circuit whereas the other constitutes a Y coordinate circuit to detect a resistance ratio at a touched point in an analog fashion, and the touched point is determined from an X coordinate and a Y coordinate where the transparent conductive films 21 are brought into contact with each other.
[Capacitive Type]
In a capacitive touch panel, a weak current runs all over a surface of a transparent substrate having a touch panel function. When a person touches this surface with his/her finger, electricity flows into the person's body, creating a change in amount of electric charge. The change in capacitance between the fingertip and a conductive film is captured and a dedicated touch controller outputs the amount of change as coordinate data.
Other touch panel types include optical type, ultrasonic type, surface acoustic wave type, induction type, and capacitive coupling type.
As is the case for a protective plate, when one of those touch panels is disposed on the front side of a liquid crystal display panel, reflection of light is caused at the interface between materials having different optical characteristics, specifically, refractive indices. In the case of a structure as that of a liquid crystal display device 12 with a touch panel which is illustrated in FIG. 6, reflection of light is observed at the interface between a transparent substrate 11 having a touch panel function and air that is existing in front of the transparent substrate 11 having a touch panel function, the interface between the transparent substrate 11 having a touch panel function and an air layer 8 located behind the transparent substrate 11 having a touch panel function, and the interface between a first polarizing plate 4 and the air layer 8. The reflection significantly impairs the visibility of a displayed image, especially in a bright environment.
In order to prevent the reflection of light due to a difference in refractive index, JP 5-11239 A and JP 2007-41534 A propose a panel structure illustrated in FIG. 7 where the air layer 8 between the protective plate 2 and the polarizing plate 4 is filled with a transparent organic medium (transparent interlayer 3) whose refractive index is equal to or close to that of the protective plate 2 and the polarizing plate 4. With the air layer 8 thus eliminated, external light is reflected less at an interface and the visibility of a displayed image is improved.
The interlayer disposed between the liquid crystal display panel and the protective plate may be made from a liquid material or a solid material. The panel structure proposed in JP 5-11239 A uses a liquid material for the transparent interlayer. JP 2007-47621 A proposes a method of manufacturing this panel structure. JP 2005-89195 A proposes a manufacturing method, though not of a display device, in which two glass substrates sandwich a liquid transparent interlayer whose refractive index is equal to or close to that of the glass material.
In order to give an example of the transparent interlayer that is made from a solid material, JP 2001-31451 A, JP 2005-187237 A, and JP 7-290647 A propose a product (laminated glass, not a display device) structured such that two glass substrates sandwich a solid sheet material whose refractive index is equal to or close to that of the glass material, and a method of manufacturing the product.
JP 7-209635 A describes a method of manufacturing an anti-light scattering structure in a liquid crystal display device in which a protective panel made of synthetic resin is framed by a frame portion, which includes a distortion tolerance area at the circumference thereof, to have a dish shape, a liquid raw material of an anti-light scattering material is injected onto the top surface of the protective panel, or a cured anti-light scattering material is inserted on the top surface of the protective panel, a liquid crystal cell is then disposed from above, and an excess amount of the anti-light scattering material is led to the distortion tolerance area by depressing the liquid crystal cell to deaerate and bond the components together intimately.
In the case of a display device structured to have an interlayer made from a liquid material between a display panel and a protective plate, or a display device structured to have interlayers made from a liquid material between a display panel and a touch panel and between the touch panel and a protective plate, the liquid interlayer material may leak out of the display device during manufacture of a product or while the product is in use. A measure to avoid the leakage needs to be taken by providing a bank for preventing the interlayer in a liquid state from overflowing outside an effective display area of the display panel so that the effective display area is surrounded by the bank.